1. Field of the Invention
The present invention relates to optical fiber cables and optical fiber cable components. More specifically, the present invention relates to processing thermoplastic polyolefins, such as polypropylene-polyethylene copolymers, to make optical fiber cable components.
2. Description of the Prior Art
Optical fiber cables have been in use in communications industries for a number of years to transmit information at very high rates over long distances. In an optical fiber cable the information is carried in the form of light signals through glass fibers with diameters on the order of 100 .mu.m. These fibers are protected from the environment and external stresses by the cable structure.
In designing the cable structure it is important to ensure that process or construction induced stresses related to cable production do not interfere with optical fiber performance. The general trend in the industry is to increase rates of production to meet demand and increase profitability by increasing linespeeds on production equipment. For extruded components such as optical fiber buffer tubes, filler rods, cores, or jackets, higher line speeds may result in greater shear rates and higher orientation and residual stress in the finished product especially if an optimal material is not used.
Optical fiber cables may be classified into three general classifications based on cable structure: loose tube, monotube, and slotted core. In loose tube optical fiber cables, the optical fibers lie in a plurality of optical fiber buffer tubes which are generally filled with some type of water blocking compound such as a gel. These loose tube buffer tubes are stranded around a central member. In the loose tube design, in addition to the buffer tubes, filler rods may be stranded around the central member in order to provide symmetry in design for fiber counts lower than that of a full fiber count cable. These filler rods may be made of solid or cellular polymer.
In a slotted core optical fiber cable the optical fibers reside in channels or slots which are generally filled with a waterblocking gel. These channels form a helical path along the longitudinal axis of the cable.
In a monotube cable the optical fibers reside in a central tube which is generally filled with some type of waterblocking compound. In all of these structures, the buffer tube or core provides the primary structure to protect the thin optical fibers contained within. Typically the buffer tubes or core is jacketed with an additional protective layer. Additionally reinforcing yarns or fibers as well as waterblocking materials in the form of gels or hot melts, water swellable powders, yarns, or tapes, and/or corrugated armor may be placed between the jacket and the inner cable layers.
For each buffer tube design, it is important to choose material combinations which are compatible in terms of basic material properties and processability. Additionally, a choice of materials and processing conditions must be chosen which results in a cable which has high compression resistance and tensile strength combined with a low amount of residual stress. It is also important to choose a combination of materials and processing conditions which has minimal changes in dimensions as a function of time and temperature. It is desirable for a material to have a low coefficient of thermal expansion (CTE) to ensure that the fibers are not placed under stress as the cable endures the high and low temperature extremes encountered within its environment. Material and processing conditions which minimize process induced orientation are also desired since these will minimize the post extrusion relaxation and shrinkage of cable components. Post extrusion shrinkage of buffer tubes can lead to an increase in excess fiber length (a ratio of fiber length to actual tube length) which can, in turn, cause increases in fiber attenuation.
Fiber optic buffer tubes or cores have been primarily made from "engineering resins" such as polybutylene terepthalate (PBT), polycarbonate (PC), a polyamide such as nylon-12, or some layered combination of the above. Generally, these materials have been chosen due to their high modulus and low CTE relative to other polymers. These materials have disadvantages compared to polyolefin based buffer tubes such as nucleated polyethylene-propylene copolymer buffer tubes which include increased cost, decreased buffer tube flexibility, decreased hydrolytic stability, and more difficult processability.
Generally, polyolefins have not been used for buffer tube applications due to a combination of reduced modulus and other physical properties relative to the above described "engineering resins". These limitations included reduced modulus, decreased compatibility with waterblocking gels, and lower dimensional stability at high temperatures. However, polyolefin buffer tubes made of a nucleated copolymer of polyethylene and polypropylene have been used. See Yang, H. M., Holder, J. D., and McNutt, C. W., "Polypropylene-polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same", U.S. Pat. No. 5,574,816. These materials have become useful for fiber optic buffer tube applications due to improvements in modulus, compression resistance, solvent compatibility and other properties brought about by the inclusion of a crystal nucleating agent in the polymer resin formulation. Since 1995, flexible polyolefin buffer tubes have become increasingly attractive from an application and installation standpoint due to greater ease in buffer tube access, handling and relative cost. See Adams, M., Holder, J., McNutt, C., Tatat, O., and Yang, H., "Buffer Tubes-The Next Generation", International Wire and Cable Symposium, 44th IWCS Proceedings, 1995, 16-21. Also, see Holder, J. and Power, R., Lightwave 1995. Increased demand for these materials has caused the necessity of increasing production capacity.
"Extrusion grade" materials of the prior art are generally characterized by a low Melt Flow Index. The Melt Flow Index (MFI) of a polymer is determined by measuring the amount of material which flows through an aperture of a fixed size during a fixed time period at a set temperature when placed under a fixed load. The melt flow index (MFI) is determined according to an ASTM Method such as D1238-57T5. This method determines MFI at a temperature of 230.degree. C., with a total weight applied of 2160 g, a die diameter of 0.0825 in., and a die length of 0.315 in. A correlation between MFI and molecular weight for polypropylene according to this ASTM method has been determined and reported in Frank H. P., Polypropylene, Macdonald Technical and Scientific, London, 1968 and has been used to determine approximate molecular weights for the samples used in this study.
Generally, low MFI materials have a high degree of melt strength and good dimensional stability of the extruded profile after exiting; the extrusion die. Additionally, general trends have been observed correlating a higher molecular weight (low MFI) to improved mechanical properties of extruded parts. As a result, low MFI materials (materials with MFI&lt;3) are generally recommended by polymer manufacturers and suppliers for extruded optical fiber cable components. A disadvantage of using low MFI materials is that due to high melt viscosities these materials develop processing difficulties at the high shear rates associated with high linespeeds. Among the processing difficulties which may be associated with these materials is increased viscous heating of the polymer melt, process induced orientation, and decreased crystallization rates.
Each process has with it a set of process conditions which establish the criteria for optimal material properties. Within an industry material choices are made based on requirements based on process conditions and product end use. Polypropylene has been used extensively in the textile industry for much longer than in the optical fiber industry and extensive research has been undertaken to investigate the effects of molecular weight (MFI) and molecular weight distribution in relation to polypropylene processing in the field of fiber spinning.
Spruiell and coworkers have done extensive work investigating the effects of molecular weight, molecular weight distribution, and processing conditions on the processing of polypropylene filaments. See Misra, S., Lu, F. M., Spruiell, J. E., and Richeson, G. C., J. Appl. Polym. Sci. 1995, Vol. 56, pgs. 1761-79; Lu, F. M. and Spruiell, J. E., J. Appl. Polym. Sci. 1993, Vol. 49, pgs 623-31; Lu, F. M. and Spruiell, J. E., J. Appl. Polym. Sci. 1987, Vol. 34, pgs 1541-56; and Lu, F. M. and Spruiell, J. E., J. Appl. Polym. Sci. 1987, Vol. 34, pgs 1521-39. In these studies, it was found that under the processing conditions encountered during fiber spinning, increased crystallinity, modulus, and tensile strength were observed for non-nucleated polypropylenes as a function of decreasing MFI, increasing molecular weight. This trend was attributed to increased molecular orientation while processing higher molecular weight (lower MFI polypropylenes) and subsequent strain induced crystallization occurring during processing. Experiments at increased line speeds and draw ratio verified this hypothesis. Comparisons were also made between nucleated and non-nucleated polypropylenes as well as between a propylene homopolymer and an ethylene-propylene copolymer. See Bodaghi, H., Spruiell, J. E., and White, J. L., Int. Polym. Process 1988, Vol. 3, pgs. 100-112. The copolymer was similar to the non-nucleated version of the material which is the subject of this invention. Under the high shear rates generally associated with optimal polypropylene fiber performance, the effect of a nucleating agent on fiber properties was found to be insubstantial. However, at lower shear rates fiber tenacity is reduced by the addition of a nucleating agent although crystallinity is modestly increased. The effect of copolymerization of polypropylene with polyethylene was found to decrease crystallinity and the rate of crystallization substantially.